Error with strictNullChecks when nested function

[horiuchi]

TypeScript Version:

Version 1.9.0-dev.20160624-1.0

Code

test.ts

function g(cb: () => Promise<boolean>): Promise<boolean> {
    return cb();
}
function f(a: string | null): Promise<boolean> {
    if (a === null) {
        return Promise.resolve(false);
    }
    return g(() => {
        console.log(a.substr(1));
        return Promise.resolve(true);
    });
}

execute the command tsc --target es2015 --module commonjs --strictNullChecks test.ts.

Expected behavior:

There is no compile error.

Actual behavior:

test.ts(9,21): error TS2531: Object is possibly 'null'.

[kitsonk]

Essentially duplicate of #9263 and #7662 as narrowing is reset across function boundaries as explained in #7718 (comment).

[horiuchi]

OK, I understand 👍